The present invention generally relates to methods for manufacturing composite laminate articles and, more particularly, to methods for applying pressure to composite laminate areas that are masked by secondary features. Composite laminate areas masked by secondary features include the area of an uncured part, such as a skin preform, that experiences reduced pressure during co-curing/co-bonding processes due to the presence of a secondary feature, such as a precured frame web.
Composite laminate materials are known in the art. They are used extensively in the manufacturing of aircraft and other lightweight structures. They have a high strength-to-weight ratio and high stiffness. As is known in the art, a plurality of plies of uncured fiber reinforced organic matrix prepregs may be placed on a shaping tool. The prepregs may then be cured using known methods, such as vacuum bag processes, to produce the desired cured structure. Dry fiber preforms are also known in the art and have also been used to produce composite structures.
Vacuum bag processes may comprise covering the prepregs with a release material. The lay-up—comprising the prepregs, tool and release layer may then be hermetically sealed within a flexible vacuum bag. The bagged lay-up may then be evacuated to remove as much air and other volatiles as possible. After evacuation, the bagged lay-up may then be subjected to elevated temperature and/or pressure to cure the prepregs. Resin infused preforms may also be cured by known vacuum bag processes.
During curing, air and volatiles within the prepregs may be removed by vacuum. The organic resin may then flow into the areas previously occupied by the air. This process may be used to make composite laminate structures, but the shape and size of the structure is limited to the shape and size of the tool. Although tool shapes have advanced beyond those used to produce flat sheets, even more complex shaped structures are desired.
Two or more composite laminate structures have been joined together to form larger more complex shaped structures. Cured composite laminates have been joined together by mechanical fastening, adhesive bonding, and thermoplastic welding, such as moving coil welding and fixed coil induction welding. Although these joining methods can be used to produce complex shaped structures, they are not useful for some applications.
Other methods for producing complex shaped structures are known in the art. U.S. Pat. No. 5,817,269 describes methods for producing complex composite structures, such as wing boxes. In the described methods, two uncured parts, such as a wing skin lay-up and a rib lay-up, may be cured simultaneously. Although the disclosed methods may be used to make complex shaped articles, they require complicated tooling techniques. The described tooling techniques may not be feasible or desirable for some applications.
Co-curing/co-bonding processes have also been used to produce complex composite laminate structures. Co-curing/co-bonding processes may comprise laying up and precuring a composite part, such as an I-beam. This precured part may then be placed on an uncured lay-up, such as a wing skin lay-up. Bond clips, such as angle clips and pi ties, as known in the art, may be used to reinforce the interface between the uncured part and the precured part. The uncured lay-up together with the precured part and angle clips may then be vacuum bagged and co-cured/co-bonded. Co-curing/co-bonding processes have been used to produce wing boxes and other complex shaped structures. Unfortunately, known co-curing/co-bonding processes result in the buckling of the uncured part into the gap at the interface between the uncured part and the precured part on relatively thin structures. This buckling may cause wrinkles in the outer mold line. Additionally, for some applications, the known processes result in an undesirable amount of un-reinforced resin at the interface or a low fiber volume within the laminate. Un-reinforced resin or high resin volume laminate reduces the structural integrity of the finished product and has a tendency to microcrack.
As can be seen, there is a need for improved methods for producing complex shaped composite structures. Also, there is a need for a method of eliminating the buckling or wrinkling of uncured laminate during co-cure/co-bond processes. Further, there is a need for a method of reducing the volume of un-reinforced resin between two co-cured/co-bonded parts.